bmpstation: make configurable through environment variables

you can specify listening HOST and PORT by setting env variable
'RYU_BMP_SERVER_HOST' and 'RYU_BMP_SERVER_PORT'.

'RYU_BMP_OUTPUT_FILE' specifes the output file name
'RYU_BMP_FAILED_DUMP' specifes the file name of the error dump

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
sw test tool: add a "note" for help to understand

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>